Because of many advantages including impact resistance, light weight, and workability, organic resin materials are used in a wide variety of applications. Efforts are currently made to take more advantage of these properties. One such approach is to apply molded organic resins having enhanced surface hardness and abrasion resistance to the windows in various vehicles. In the glazing application, a high level of abrasion resistance and outdoor weather resistance comparable to glass are required. In the case of automobiles, for example, a high level of abrasion resistance is required in order to prevent the windshield from marring upon wiper operation and to prevent side windows from marring upon winding up-and-down operation. Potential service in a very high temperature and/or humidity environment must also be taken into account.
In the prior art, substrates of organic resins or plastics are surface coated with various coating compositions to form surface protective films for the purpose of imparting high hardness and mar resistance. For instance, compositions comprising hydrolyzates or partial hydrolyzates of hydrolyzable organosilanes and optionally, colloidal silica are known.
For instance, JP-A S51-2736, JP-A S53-130732 and JP-A S63-168470 disclose coating compositions comprising an organoalkoxysilane, a hydrolyzate and/or partial hydrolyzate of the organoalkoxysilane, and colloidal silica, wherein the alkoxy group is converted into silanol in the presence of excess water. However, these coatings resulting from wet coating systems suffer from problems of low hardness and poor mar resistance as compared with glass or the object to be replaced.
Several problems must be solved before coating films can withstand sunlight and weather over a long time. The wet or dry coating layers having mar resistance lack an ability to cut UV, and a phenomenon develops that a resin substrate, a primer layer for imparting substrate adhesion or an interface therebetween can be degraded or discolored by UV exposure. Several techniques are proposed to prevent such a phenomenon, including addition of UV absorber to the primer layer, and incorporation via chemical bonds of UV absorptive organic substituent groups into the organic resin of which the primer layer is formed. The UV absorptive organic substituent groups and UV absorbers refer to benzophenone, benzotriazole, triazine and similar substituent groups, and organic compounds containing the same. See JP-A H04-106161, JP 3102696, JP-A 2001-47574, and JP 3841141.
The above technique for cutting off UV is by incorporating an organic UV absorber into a primer layer. Since the primer layer in itself has the main purpose of improving the adhesion between the underlying substrate and a silicone layer, an extra amount of UV absorber loaded gives rise to problems such as losses of adhesion and transparency. It is demonstrated in a long-term outdoor exposure test and accelerated weathering test that the UV cut by the primer layer alone is insufficient for preventing degradation and discoloration of organic resin substrates.
One approach taken for compensating for such drawbacks was to add organic UV absorbers to silicone layers as well. However, simply adding such compounds to coating compositions results in a coating lacking durability. That is, the coating fails to sustain the desired UV absorbing property due to bleeding and drainage of UV absorber from the surface during long-term weather exposure. Then organic UV absorbers were developed which are silyl-modified so as to be chemically bondable with siloxane compounds, the main component of the coating layer. See JP-B S61-54800, JP-B H03-14862, JP-B H03-62177, and JP-A H07-278525. This measure improves retentivity since the UV absorber is strongly bound to the siloxane matrix. On the other hand, these coating layers become substantially poor in mar resistance that is essentially desired, or develop noticeable microcracks due to a lowering of flexibility. As discussed above, the organic UV absorbers have the essential drawback that the hardness of silicone film becomes lower as the amount of UV absorber added is increased to enhance weather resistance.
As discussed above, the wet and dry coating system is successful in imparting a high level of weather and mar resistance, but requires a multilayer coating process, with an urgent need for simplification of the multilayer deposition process to reduce processing time, increase yield, and ultimately reduce cost. The practical coating system is composed of at least four layers, organic resin substrate, primer layer, weather resistant layer, and dry abrasion resistant layer. Herein, the primer layer is mainly formed of acrylic coating, while the weather resistant layer is mainly formed of silicone hard coating. The system is produced by coating and curing a primer wet coating to an organic resin substrate, coating and curing a silicone hard coating thereto, and further forming an oxide coating by a dry process.
As discussed above, a number of attempts have been made to improve the weather resistance, mar resistance and other properties of wet and dry coating films. However, there is not available a laminate having a coating system which exhibits visible light transparency and UV shielding property, and sufficient weather resistance and durability to withstand prolonged outdoor exposure while maintaining a very high level of mar resistance (i.e., comparable to glass), the laminate being manufactured in a simple manner